Mission specialist Rex Walheim waves in Atlantis' payload bay during a spacewalk in this image from NASA TV February 13, 2008.

WASHINGTON, Feb. 13 (Xinhua) -- Two astronauts completed the second of three scheduled spacewalks at 4:12 p.m. EST (2112 GMT) on Wednesday, replacing a nitrogen tank outside the International Space Station, according to NASA TV.

Space shuttle Atlantis' astronauts Rex Walheim and Hans Schlegel floated out of Quest airlock at 9:27 a.m. EST (1427 GMT).The duo successfully removed an expended nitrogen tank outside the station and installed a new one. The tank is part of the orbital outpost's cooling system.

U.S. astronaut Walheim, a veteran spacewalker, took part in the mission's first spacewalk on Monday. For Schlegel, a German astronaut representing European Space Agency, this is his first spacewalk after sitting out Monday's spacewalk due to an undisclosed illness.

On Tuesday, he told media on the ground that he was feeling better and preparing for Wednesday's spacewalk. He proved himself during today's nearly-seven-hour excursion, looking and sounding fit.

Their service call to the station lasted six hours and 45 minutes. With the new tank installed, the old tank was transferred to Atlantis' payload bay for return to Earth.

Because they finished their primary tasks early, the spacewalkers were able to install thermal covers outside the station's new European Columbus laboratory. They also inspected and adjusted the U.S. Destiny laboratory's orbital debris shields.

Mission Specialist Stanley Love will join Walheim for the third spacewalk on Friday. They will install two payloads on the exterior of the Columbus lab.

European-built Columbus lab was delivered to the station by U.S. shuttle Atlantis, which lift off on Feb. 7 after a series of delays. The new module is about 4.5-meters wide and adds an extra 75 cubic meters of breathing room aboard the station.

Columbus is built to hold a total of 16 equipment racks, 10 of which will be devoted to scientific research. The European Space Agency has set up a new control center near Munich, Germany to oversee the module's daily operations.

NASA's space shuttle Mission Management Team, at the request of the International Space Station Program, has extended Atlantis' mission from the original 11 days to 13 days, NASA announced at its official website following the completion of the second spacewalk.

According to the updated schedule, Atlantis will undock from the space station on Monday, Feb. 18, and land at 8:59 a.m. EST (1359 GMT) on Wednesday, Feb. 20, at Kennedy Space Center, Fla.